SSM perspective
Background and objective
Large earthquakes occur predominantly along plate boundaries. Due to this, much of the research have been directed towards this tectonic setting. Also, the general lack of seismicity data in stable continental interiors, such as the Baltic Shield, have impeded estimations, as well as the understanding, of the seismic hazard in these tectonic settings. Large earthquakes at plate margins tend to be stationary phenomena recurring at the same fault system. Observations of the recent seismicity and paleoseismic records in intraplate settings suggests a more migrating earthquake pattern (Stein et al., 2009). It has even been argued that earthquakes in stable continental interiors can occur in regions with no previous seismicity and no surface evidence for strain accumulation (Calais et al. 2016).
The proposed project will amalgamate information concerning the present understanding of the stationarity of earthquake occurrence in intraplate tectonic settings and maximum possible magnitudes. It will also compare the shear displacements on target fractures predicted by SKBs rock mechanics approach, to observed distributed fault displacements in historic earthquakes. In this comparison an assessment shall be made about the relevance of this comparison with the Forsmark site regarding the tectonic setting, geological complexity, recent seismicity and paleoseismic records. These questions are of importance for the safety analysis of radioactive waste repository since they can be used to check the reliability of SKBs assumptions regarding secondary shear displacements in relation to the distance to the primary shearing event and the presumed long term stability of the Forsmark site.
Results and conclusions
Based on recent work by the Scandinavian geological surveys, SKB and POSIVA, we now see a pattern of large-magnitude seismicity from today back to the end of the middle Weichselian (ca. 57 ka). It appears that surface ruptures have remained in the same areas during this time, which suggests spatial stationarity and not unpredictable migration.
Digital Surface Rupture Databases containing rupture maps and dis-placement measurements, and recently-published statistical analyses of off-fault displacements were used to predict off-fault displacements as a function of distance from the Principal (coseismic, activated) fault. The prediced off-fault displacements were compared to numerical displacements, for the same earthquake magnitude and distances from the fault.
Nurminen et al. (2020) and Moss et al. (2022) (for reverse faults) and Petersen et al. (2011) present equations for distributed displacement as a function of magnitude and distance. In every case their predictions are larger, usually much larger, than the target fracture displacements predicted by SKB using the 3DEC software. Compared to the displacements predicted by Yoon et al. (2014) and Yoon and Zang (2019) using Particle Flow Code 3D v4, the empirical displacements were a closer match. The closest match was between empirical displacements and Particle Flow Code -predicted displacements on other named faults (deformation zones, not smooth fractures). This suggests that most empirical displacements in the databases represent reactivation of pre-existing deformation zones, not of smooth fractures. If this is the case, then there is no conflict between the (larger) empirical and (smaller) numerical displacements, because they are measuring different phenomena.
Recommendations
Issue 1-Establish exactly how PFC modelling of induced displacement on other DZs (which matches empirical displacements) differs from PFC modelling of induced displacement on smooth target fractures (which underestimates empirical displacements). Is it because the DZs were assigned different geotechnical properties than the smooth joints in the PFC model? And if not that, what causes the difference?
Issue 2-Having answered Issue 1, can 3DEC modelling be similarly reconfigured to output induced displacements on DZs more in line with empirical displacements?
Issue 3-When additional PGFs are mapped in Sweden using lidar, investigators should look for possible DFs associated with the main PGF scarp. If trenching is performed, on the PGF scarp, consider also trenching the possible DF scarps. Some care should be taken: (1) lengthen the trenches away from the PF scarp to look for evidence of DFs which might have been obscured by weathering and erosion, and (2) document the type of bedrock structure that underlies any DFs. The more surface rupture data we can obtain from the Fennoscandian SCR, the less we will have to rely on analogs from other SCRs (such as Australia), which possibly might not be appropriate for use in Fennoscandia.